Often in a microcomputer system, a large amount of data needs to be transmitted between data input/output equipment such as a CRT terminal, a hard disk or a floppy disk, and a memory, and between memories. When a microprocessor is charged with such data transmission by the software, the load on the microprocessor is greatly increased; consequently, the effective capacity of the system decreases.
To cope with this disadvantage, a DMA (direct memory access) controller has been furnished as one item of equipment for controlling data transmission between component elements of the microcomputer. However, since a conventional DMA controller is an LSI (Large-sized integrated circuit) formed separately from the microprocessor, the use of a conventional DMA controller is disadvantageous in that it complicates the construction of a system for DMA transmission. That is, it becomes necessary to mount the microprocessor LSI as well as the DMA controller LSI on a printed wiring board. Further, it is often required to provide a suitable logic IC such as TTL to form an interface between the microprocessor and the DMA controller. This fact causes the construction of board such as wiring substrate to become complex.
The present inventors thought about simplifying the construction of the system by incorporating the function of a DMA controller in a microprocessor LSI. The mere incorporation of the DMA controller in the conventional microprocessor however, markedly increases the chip size of an LSI, thereby creating a possibility of lowering the production yield.
One DMA controller, and the microcomputer system in which that controller is used, are described in the Aug. 2, 1982, issue of "Nikkei Electronics", published by Nikkei McGraw-Hill, No. 296, pp. 129 to 158.